'Your heart, my heart, everyone's heart, is a noisy room full of voices. These voices can come deep from childhood, or from our culture, or from habits embraced in adulthood. These voices run our lives. They can even ruin them. But if we face them, they can also save us' (Ronald Boyd MacMillan, The Heart is a Noisy Room).

What is a voice? Where might it come from? Why is it so powerful? How might we say it actually has the power to run our lives?

When I first started teaching on this, this was my first pass on the subject. I defined a voice this way: 'A voice is a persistent, powerful message, from yourself to yourself, that prevents and keeps you from embracing your true self.'

The idea here was to show these things through the elements of this definition:

Persistent, that is, it's part of your identity. It feels familiar. Even if it is never named, it is a companion over time.

Powerful. We tend to believe it. We rarely question its truth, and we have an emotion every time the voice goes off.

Message. It's a declaration or assertion, emerging from the subconscious in most cases. It's asking us to take a point of view – I am rubbish, or, I am invincible.

From yourself to yourself. The voice is within, internal, rarely spoken out, but you often find yourself in dialogue with it.

Prevents and keeps. These voices are trying to prevent you from getting what you really need more deeply. They are substitutes of lesser identities for who you really are.

True self. Our deepest identity is to love, know and serve God, right? Every Christian affirms that, at least theoretically...

Most voices probably do come from ourselves, but since we are people who believe in the sacred, there are two other sources of the voices that are quite external to us – God and the pit. Whether you take the devil metaphorically or not, evil does seem to have been allotted the power to suggest, and to take those suggestions into our very hearts. Jesus famously duels with the devil in the wilderness, and although we are not told what form the devil's voices took, the least we can infer is that they entered Jesus' consciousness, whether or not the devil was sitting on a rock with a tail, antlers and red from sunburn. But there was another problem with my first definition. It didn't really describe what a voice was like.

So here's my second pass: 'If you have (A) streams of sentences in your head, (B) that recur frequently, (C) which seem to have a distinct perspective, (D) and that carry an emotional feeling with them, you've got voices.'

Yep, that's a more technical definition of what a voice actually is. It's important to see what a voice is not. It does not have to be:

• audible
• loud
• overwhelming
• sacred-sounding

Nor does it have to come using a particular pronoun, like 'You' or 'I'. Nor does the voice have to be experienced as a particular mini-person, and indeed some good research shows that the voice can form without extensive sentence streams, as our brains can use shorthand to convey the perspective. Martin Laird, in his beautiful book A Sunlit Absence, has a chapter called 'Our Collection of Videos', and he defines them as:

'...inner chatter...something like a video that constantly plays in the mind only to be rewound and played again and again and again. For some it might be a predominantly visual sequence of distractions, for others predominantly aural, or a combination of both. The insidious thing about these videos is that they have a way of cultivating a psychological identification with them.

What he's calling our videos I suppose I am calling our voices. But put in these ways, most of us have them. In fact, we'd be a bit weird if we didn't.

So much for the technical, I hear you yawn. It's the actual power I want to talk about, and this is my view, that these voices are trying to shape our identities, and the less attention we pay to them, the more our personality is shaped wrongly. If we pay attention to them correctly, we have a chance of being the person God wants us to be. They are trying to label us. It feels like a person is inside us that isn't us. These labels matter. 'You are a waste of space,' 'Shut up, you've nothing to offer,' 'You will always be defective.'

...I remember the day when I won a spelling competition in primary school. Quite an achievement I may say, even though I don't think we had any Einsteins at the primary school I was attending. I ran all the way home in great excitement and showed it to my mother. She smiled, but added, 'Dinna go get'n a big heid.' That's broad Scots for 'Don't go getting a big head.' A little crestfallen at this muted reaction, I ventured to point out what a singular, world-beating honour this was. She was moved to add, 'Who do think you are? Just suffer the word of exhortation.' Stern rebukes always came in the language of the King James Version.

Now, my mother wasn't being ungracious. She loved me, but she was Scottish of a certain generation, wary of all notions of pride, where all celebration was premature celebration. She just wanted to keep me from getting above my station. But when that notion gets glued on to a church theology where no one deserves grace, and everyone is falling short, you end up with an embedded 'undeserving voice' that intrudes even on your very intimate times with God.

Say you are reading Isaiah 49:16: where God is saying, 'See, I have written your name on the palm of my hands' and you just begin to take that in when Undeserving Voice pops up to say, 'Hey, that's God speaking to Israel, not you. Don't get ahead of yourself.' Or you are reading the annunciation passage, and the angel Gabriel says to Mary, 'Greetings, favoured woman.' In The Message paraphrase the sense of it is caught really well: 'Good morning, you're beautiful with God's beauty. Beautiful inside and out! God be with you.'

Ah, what a message. So you start to think, 'Wow, I'm as beautiful as Mary in the sight of God. We're all equally valuable in God's sight.' And just as you set yourself to receive that compliment from God, Undeserving Voice pops up and says, 'Wait a minute. You're no Mary. She was pure. You've got a few things on your conscience, haven't you?' And that compliment from God to you, that 'you are favoured, you are beautiful!' gets snatched away. Undeserving Voice won't let you receive it.

And that's just the way one single voice snatches our joy, or hurts a culture, or rips the heart out of the gospel in a church experiencing the world's largest revival in the history of Christendom. The church of 1001 commandments.

Voices. They matter. They matter an awful lot.

Extracted with permission from 'The Heart is a Noisy Room'by Dr Ronald Boyd-MacMillan, published by Hodder and Stoughton.

Ronald Boyd-MacMillan is director of strategic research for Open Doors International, the world's largest organisation assisting persecuted Christians worldwide, and a Professor of Practical Theology in Pakistan. 'The Heart is a Noisy Room' is his third book.